Method and plant for controlling the colonization of submerged structure surfaces by aquatic filtering organisms

ABSTRACT

The present invention relates to a method and to a plant for controlling and/or preventing the colonization of submerged structure surfaces by aquatic filtering organisms based on subjecting waters near said surfaces to at least one biocide treatment, where the provision of the biocide treatment is based on a monitoring to identify phytoplankton biomass values and on a calculating of a period of maximum density of the aquatic filtering organisms.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to the control and/or the prevention of the colonization of submerged structure surfaces by aquatic filtering organisms.

More specifically, the present invention relates to the control and/or the prevention of the colonization by aquatic filtering organisms that may foul submerged conduits of industrial plants.

PRIOR STATE OF THE ART

Large-sized power plants and different greater coastal industries use enormous amounts of water for steam generation, as well as for turbine running and general service.

The greatest water source is represented by sea, lakes and rivers, yet the content of algae, bacteria and microorganisms causes serious problems due to the fact that plants are fouled: also larvae and young individuals of plankton species, like cirripeds and mussels, may be let in with the water, and their colonization in the inside of the exchangers causes a decrease in the effectiveness and finally the complete shutdown of the plant itself.

In these plants, control of the fouling due to aquatic organisms has always been related to the use of chlorine: historically, its application, in a continuous or discontinuous manner, has been the most widely adopted choice, above all to fight microbiological slime.

However, it is known that to control a fouling due to largish-sized aquatic organisms the discontinuous use is often ineffective, whereas a continuous addition has often been selected, even at lower dosages.

The advantages of chlorine use as control method are essentially related to its very low price. However, from the early '70s, organic chlorine by-products had been identified in effluents during water chlorination, and a few toxicity assays sufficed to highlight the high environmental hazardousness of these by-products, even at concentrations of a few parts per billion (ppb).

Over the last 20 years, the environmental impact and public hygiene implications due to chlorine-based treatments drove the quest for a valid alternative: some methods were introduced, based on chemical as well as on physical alternatives.

Physical methods comprise the use of ultrasounds, thermal shocks, UV light, filters, powders or abrasive means, etc. Actually, each of these methods entails marked drawbacks or is physically inapplicable to systems on which a control is required, and this has prevented their widespread use.

Biocides alternative to chlorine are ozone, bromine, bromine chloride, hydrogen peroxide: the high cost and the uncertainty on the toxicity of the by-products of these biocides (bromine organic derivatives, ozonides, peroxides and epoxides), have quelled the drive towards these alternatives (U.S. Pat. No. 5,393,781), actually increasing coastal industries' dependence from chlorine use as a method for controlling the fouling due to aquatic organism colonization.

In the US, the emergency due to the invasion of Zebra mussels (Dreissena polymorpha), has roused interest towards alternatives to chlorine: moreover, the heightened attention to water quality, in the US as well as in Europe, above all for lakes and rivers, caused an expectation of reduced release of chemical products into natural ecosystems.

Recently, the Canada-US International Joint Commission recommended the banning of chlorine production and of its use in the Great Lakes basin. It is evident that the non-chemical alternatives to chlorine are greatly favoured over the use of chemical products: however, to date the methods advanced are often deemed ineffective and the supporting data too vague and void of specific validity.

Concerning the manner in which these biocides are released in the environment, the commonly adopted approach is the ‘continuous’ one, in which the biocide is continuously released in the system and hence into the environment. In order to limit the introduction of pollutants into the environment, a ‘discontinuous’ system may be adopted, foreseeing an increase of the biocide dose introduced in the environment, but at subsequent times (e.g., about 15 min per hour, hence, only 6-8 hours per day). This latter approach allows to increase the dosage, rendering it more effective against the more resilient species (e.g. mussels) without however dramatically increasing the amount of biocide introduced in the system, and, consequently, released into the environment.

However, it being known that the greater probability of introduction of filtering organisms, like e.g. mussels—and in general of undesired biological species in submerged conduits—subsists in the spring-summer period, the actual technology for the treatment of said structures foresees a lower wintertime dosage and a higher summertime dosage, concomitantly to the temperature rise. Such an approach, though generally functional, allows no ‘selective’ adaptation for all those events of colonization by said organisms, related to atypical climatic-environmental conditions like sudden temperature rises, etc.

In the light of the above, it appears that the problem of reducing the pollution and the waste of chemical products apt to control the colonization of filtering organisms onto submerged surfaces is still unsolved. Although there have been improvements over the last 30 years, to date said chemical products are still released into the environment abundantly and quite arbitrarily, thereby not allowing to abate the release of said substances into the environment to the bare minimum required.

Hence, scope of the present invention is to solve this problem and to provide a method, and a suitable apparatus, for controlling and/or preventing the colonization of submerged structure surfaces by aquatic filtering organisms, abating to the bare minimum the release of chemical products having a biocidal function into the environment.

BRIEF DESCRIPTION OF THE INVENTION

Phytoplankton comprises all the unicellular microalgae that by virtue of their photosynthetic activity are capable of synthesizing organic matter, using mineral salts and carbon dioxide. Hence, phytoplankton is generally considered to be the first link in the alimentary chain (primary producers), constituting the alimentary base for a multitude of organisms belonging to the zooplankton as well as to the benthos. The maximum peak of phytoplankton abundance (called ‘phytoplankton bloom’) generally occurs, to a greater extent, at the arrival of spring and, to a lesser extent, at the arrival of autumn. E.g., in the Mediterranean Seas said peak is observed to a greater extent in the first months of the year (February-March) and to a lesser extent in autumn time (October-November). Both the adult individuals and the larvae of several filtering bivalves (like e.g. mussels), denominated veliger, feed on phytoplankton and on organic particles suspended in the water column. Zooplankton, to which there belong, e.g., Mytilus larvae, feeds on phytoplankton and hence finds the greatest availability of food just from this time on. Therefore, following the phytoplankton bloom, and after a constant latency period, a great growth of the zooplankton (called ‘zooplankton bloom’) occurs. It is evident that the zooplankton response will be all the more intensive the higher it will have been the food availability for the organisms constituting it. Hence, in general it may be stated that, over the year, the quantitative variations of the zooplankton follow those of the phytoplankton, so that consequently to the algal bloom the zooplankton will reach its maximum quantitative development. Accordingly, it is possible to foresee the time and the intensity of the colonization process (that follows the zooplankton bloom) keeping under control the phytoplankton presence, i.e. determining the time-pattern of the phytoplankton biomass, in the water column. The time-pattern of the phytoplankton biomass allows to set the best times for using the biocide for the partial or complete eradication of the infesting filtering organisms at the benthos stage (i.e., not free-swimming like the veliger larvae anymore but anchored to the submerged surfaces of interest).

Said pattern may be extrapolated following a monitoring, carried out through a sampling of the waters of interest, of the identification parameters of phytoplankton biomass values like, e.g., a phytoplankton cell count and a biomass estimate through a calculation of the average cellular biovolumes or through an evaluation of the concentration of the photosynthetic pigment chlorophyll-A.

The present invention is based on the surprising possibility of applying said zoological notions to a wholly distinct technical field, i.e. to the industrial one. It follows that a mere monitoring detecting the time-pattern of the phytoplankton biomass can allow to accurately calculate beforehand the pattern of the zooplankton population, comprising the aquatic filtering organisms infesting the waters near potentially colonizable submerged surfaces. Hence, said forecast allows to foresee the period of maximum density of said aquatic filtering organisms in the waters of interest. The accurate assessment of the period at which a maximum density of said organisms will occur in the zones of interest enables a targeted use of biocides. In fact, while to date the biocides are used indiscriminately and nonselectively as indicated above, the use of the method and of the means described in the present application allows to limit the release of biocides into the environment, by one or more treatments, at the periods, calculated beforehand, of maximum density of said organisms. Therefore, this limiting of the biocide treatments allows a drastic abatement of water pollution and a remarkable saving of the chemical products employed.

Hence, the present invention solves two problems; the environmental problem concerning the chemical pollution and the entailed damages to the ecosystem and the economic problem associated to the excessive consumption of biocides.

A further advantage of the present invention is that of being capable of acting, in a targeted manner, also in all those cases in which atypical climatic-environmental conditions may lead to sudden and unexpected blooms of undesired aquatic filtering organisms.

Moreover, the present invention, providing a forecast of the growth curve of said filtering organisms, allows the use of biocide agents at the life stage of development (e.g. at the individuals' size) in which said organisms are more sensitive to the biocides used, further enhancing the effectiveness of the latter. This allows to reduce the biocide (or biocide mixture) doses used, since the biocide may be used in a targeted manner at the time of greater effectiveness.

Another advantage of the present invention is that of allowing correction of the phytoplankton data monitored by salinity value monitoring concomitantly to that of the parameters identifying the biomass values, in marine or brackish environments. This since in waters possibly affluxed by streams of fresh water (it also containing phytoplankton) sudden changes could occur in the monitored phytoplankton values due to the sudden inlet of freshwater phytoplankton organisms. Said organisms, though temporarily constituting a food source for zooplankton, not surviving in salty waters do not really contribute to the pattern of the phytoplankton biomass of interest. Therefore, a monitoring of the salinity of the sampled waters allows to correct the monitored phytoplankton values and to attain a correct phytoplankton biomass pattern.

Hence, object of the present invention is a method for controlling and/or preventing the colonization of submerged structure surfaces by aquatic filtering organisms, comprising the following steps:

-   a. monitoring the parameters identifying phytoplankton biomass     values in waters near said surfaces; -   b. determining from said values the time-pattern of the     phytoplankton biomass and identifying a time instant at which said     pattern exhibits a peak, and; -   c. monitoring zooplankton in said time instant and assessing the     presence or the absence of zooplankton-stage larvae of said     filtering organisms near said surfaces and, in the case of presence     of said larvae; -   d. calculating as a function of said time instant a period of     maximum density of said filtering organisms; -   e. subjecting said waters to one or more primary treatments with a     biocide or a biocide mixture during said time, and, optionally, -   f. subjecting said waters to one or more secondary treatments with a     biocide or a biocide mixture during said periods of minimum density; -   g. carrying out further steps of monitoring the zooplankton in said     waters.

Object of the present invention is also the above indicated method in which the monitoring at a. may be carried out by a manual sampling and a chemico-analytical analysis of the waters of interest, directly monitoring the waters automatically or by a combination of manual sampling and automatic monitoring. Said sampling may be apt to monitor a step of metering the concentration values of the photosynthetic pigment chlorophyll-A, in which said values can be converted into phytoplankton biomass values and in which, when the waters of interest are marine or brackish, concomitantly to the sampling at a. the salinity values of the waters of interest are sampled in order to correct the values monitored at a.

Object of the present invention is also the above cited method, in which the automatic sampling is carried out by a fluorescence probe.

A further object of the present invention are the above indicated methods in which the primary treatments with biocides at e. are carried out from 1 to 8 times, for a duration ranging from 12 to 240 hours, at a biocide (or biocide mixture) concentration ranging from 300 to 0.5 ppm (mg/l) and in which the secondary treatments, at f., when carried out, are carried out from 1 to 8 times, for a duration ranging from 12 to 240 hours at a biocide (or biocide mixture) concentration ranging from 80 to 0.5 ppm.

A further object of the present invention is a plant for controlling and/or preventing the colonization of submerged structure surfaces by aquatic filtering organisms, comprising:

-   a. means for monitoring the parameters identifying phytoplankton     biomass values in waters near said surfaces; -   b. means for processing said values; -   c. means for qualitatively and quantitatively monitoring the     zooplankton near said surfaces; -   d. operating means apt to carry out treatments of said waters with     biocides.

Said plant may further comprise units for collecting and managing the data obtained by the monitoring at a. and processing means apt to integrate the values processed at a. to the data obtained by the monitoring carried out with the means at c., and, optionally, apt to trigger the operating devices of d.

Said data collecting and managing unit may be apt to apt to automatically receive said data.

Object of the present invention is also a plant in which the means indicated at a. comprises a probe apt to monitor chlorophyll-A concentration values, which may be a fluorescence probe and, optionally, a probe apt to monitor water salinity values. A further object of the invention is a plant comprising processing means apt to process the values monitored at a. into corresponding phytoplankton biomass values, to determine their time-pattern and to identify a time instant in said pattern at which the latter has a maximum or a minimum peak. A further object of the invention is a plant in which the further processing means are apt to calculate, as a function of the time instant at which the pattern of the phytoplankton biomass has a maximum or minimum peak and of the data obtained by the monitoring carried out with the means at c., periods of maximum or minimum density of said filtering organisms.

The processing means used in the above described system may comprise one or more elaborating computer programs.

DETAILED DESCRIPTION OF THE FIGURES

FIG. 1 is a graph illustrating the relationship between the phytoplankton biomass pattern expressed in mgC/m³ (mg organic Carbon/m³) (solid-line curve) and the zooplankton density pattern expressed in individuals/m³ (ind/m³) (broken-line curve). In the Figure, there are indicated also the time of the primary treatments with biocides (corresponding to the zooplankton density peak, indicated as primary shock steps) and two possible times of secondary treatment with biocides (indicated as secondary shock steps).

FIG. 2 shows a data interpolation indicating the pattern of: algal biomass (diagonal lines), herbivore biomass comprising Mitylus larvae (vertical lines), primary carnivore biomass (diagonal hatching) and secondary carnivore biomass (lattice) as a function of time. All the biomass values are expressed in mgC/m³ (mg organic Carbon/m³).

FIG. 3 illustrates a possible filtering system for the real-time observation of Mytilus veliger larvae in water.

FIG. 4 is a block diagram of a plant for preventing the colonization of submerged structure surfaces by aquatic filtering organisms according to the present invention.

In particular, a data collecting and managing unit supervises the general operation of the plant.

Monitoring means are provided to meter specific parameters in waters near the surfaces to be controlled, such as e.g. the pigment chlorophyll-A concentration, the water temperature and/or its salinity.

The metered data are collected in the collecting unit, which also collects data from further zooplankton monitoring means.

Processing means, linked to the data collecting unit, are advantageously provided for processing the collected data, in order to obtain the desired results. Said processing means comprises software procedures and programs, apt to implement specific steps of the method according to the present invention, in particular the steps of processing the metered data and of estimating the periods of maximum density of the filtering organisms.

Moreover, the data collecting and managing unit is apt to control operating means linked thereto, apt to automatically or semiautomatically carry out the water treatments required to avoid filtering organisms proliferation.

FIG. 5 (panels a. and b.) shows the results of a monitoring of Chl-A, zooplankton veliger larvaes and temperature (T) in seawater nearby an industrial plant for a one-year period time. Chl-A and temperature values were measured with an Multiparameter probe ver.0.73 (IP031A)-IDROMAR TIME DELAYED. Panel 5 a. shows the number of veliger larvae/liter of sea water (left axis of ordinates, bar graph) and the values of Chl-A measured by the probe in micrograms/liter (right axis of ordinates, linear graph) against time measured in months (axis of abscissas) . The Chl-A is monitored in a continuous mode (with intervals of two hours) by the probe. From the graph the delay of the larval peak with respect to the Chl-A peak is evident. Panel 5 b. shows the number of veliger larvae/liter of sea water (left axis of ordinates, bar graph) and the values of Temperature measured by the probe in micrograms/liter (right axis of ordinates, linear graph) against time measured in months (axis of abscissas). The temperature is monitored in a continuous mode (with intervals of two hours) by the probe.

A more detailed description of the specific components of the plant will be given hereinafter.

DETAILED DESCRIPTION OF THE INVENTION

Aquatic Filtering Organisms.

The aquatic filtering organisms according to the present invention are organisms possessing a zooplankton dispersion phase in water column, followed by a life-cycle stage competent for the colonization on a hard substrate, among which there are: bivalve, gastropod molluscs, crustacea cirripedia balanomorpha, serpulid Polychaeta annelids, encrusting bryozoans, Porifera and Ascidiacea In an embodiment of the present invention, said organisms belong to the bivalve class. In a particular embodiment of the present invention, said organisms belong to the Mytilus genus.

Submerged Surfaces

The submerged surfaces according to the present invention may be surfaces of coastal industrial plants on shores of seas, lakes, basins or rivers. Said surfaces comprise all those conduits in which the colonization of the aquatic filtering organisms according to the present invention may create fouling problems. In an embodiment of the present invention said surfaces are heat exchangers of coastal industrial plants, water inlets, and intermediate basins and tanks of industrial and non-industrial plants, inside which the environmental conditions facilitate colonization by aquatic filtering organisms according to the present invention.

Monitoring Modes of the Identification Parameters of Phytoplankton Biomass Values.

According to the present invention, the monitoring of the identification parameters of phytoplankton biomass values may be carried out manually as well as automatically. In an embodiment of the present invention, said monitoring is carried out manually by sampling, more or less weekly, waters near to the surfaces of interest, and by subsequent chemico-analytical analyses of said waters to determine the phytoplankton component. According to a further embodiment of the present invention, said monitoring is carried out continuously using automatic monitoring means, like, e.g., a fluorescence probe. The fluorescence probe operates round-the-clock, analysing at 30 min intervals for a 30 sec period with a 1 Hz frequency (one analysis per second; according to the preferred setting of the probe). Said probe may also be programmed with different parameters known to a person skilled in the art. The monitoring means according to the present invention are all those known to a person skilled in the art apt to detect the abovementioned identification parameters of the phytoplankton biomass values.

Identification Parameters of the Phytoplankton Biomass Values and Monitoring Thereof.

The identification parameters of the phytoplankton biomass values according to the present invention are all those parameters known to a person skilled in the art. Said parameters may be: biomass assessment by calculating the average biocellular volumes following the count of the phytoplankton cells, of the concentration values of the pigment chlorophyll-A (Chl-A) and of others known to a person skilled in the art.

An identification parameter is given, e.g., by phytoplankton cell count and subsequent biomass assessment by calculating the average biocellular volumes (see methods).

In an embodiment of the present invention, the identification parameters of the phytoplankton biomass values are obtained by metering the Chl-A concentration values. Said metering may be carried out according to methods known to a person skilled in the art like, e.g., chemico-analytical methods allowing a direct Chl-A metering in water samples, the use of a fluorescence probe or a combination of both. In fact, Chl-A concentration is directly proportional to the amount of phytoplankton cells in the water.

Chemico-analytical methods according to the present invention comprise, yet are not limited to, the spectrophotometric method, the fluorimetric method, the high-resolution liquid chromatography (HPLC), the fluorimetric method on suspensions in vivo (see experimental procedures).

The metering may be carried out by a biochemical analysis of the actual in-water Chl-A concentration for the conversion to quantitative estimates.

In a further embodiment of the present invention, said metering is carried out by fluorimetric method on suspensions in vivo, foreseeing no extraction procedures and carried out, e.g., by fluorimetric probe or by fluorescence probe (see experimental procedures).

In an embodiment of the present invention, the fluorescence values provided by the probe may be checked, and optionally corrected, with values obtained by a biochemical analysis of the actual in-water Chl-A concentration. The collection of samples of the water of interest, concomitant to the metering carried out with the probe, allows to even more accurately extrapolate the time-pattern of the phytoplankton biomass.

In fact, besides from an accurate probe calibration by calibration curves with highly pure Chl-A standards, a periodic checking (at least monthly) of the conformity of this metering to the actual Chl-A concentration obtained by dosages on acetone extracts with the above methodologies allows an optimal sampling.

Relationship between Fluorescence Values and Chl-A Values

The relationship between the fluorescence values and the Chl-A values is of linear type, according to an equation such as:

-   -   y−mx+q     -   where y=fluorescence (dependent variable)     -   x=Chl-A concentration     -   m=angular coefficient (slope)     -   q=offset

The fluorescence probes are calibrated for the m and q values by the manufacturer; hence, the unknown x is easily derivable from the equation.

Zooplankton Monitoring and Means for Carrying out said Monitoring.

Zooplankton monitoring according to the present invention may be carried out, in waters near the submerged surfaces of interest, at any time of the method object of the invention, besides at the time indicated at d. of the method as described. The monitoring carried out at c. of the method allows to use the biocides or the biocide mixtures of the present invention in an even more targeted manner.

Said monitoring should be carried out at intervals ranging from 24 to 72 hours. The checking of the presence or of the absence of zooplankton larvae when subsequent to the sampling of the phytoplankton biomass values, to the determination of the time-pattern of said biomass and to the identification of a time instant at which said pattern exhibits a peak, allows to decide whether to carry on with d. e. and f. of the above described method or, in the absence of larvae, to avoid carrying on with the biocide treatment. Hence, said monitoring allows to carry out treatments with biocides solely when the former detects the presence of the larvae of the filtering organisms of said invention. The density growth periods of said organisms, as a function of the time-pattern of the phytoplankton biomass and of the peak exhibited by said pattern (i.e. the delay in the appearance of the zooplankton bloom with respect to the phytoplankton bloom and the developing periods of said organisms), depend on the organisms detected during the monitoring. Said organisms and also their life-cycle being known in literature, the calculation of the period of maximum density of said organisms may easily be carried out by a person skilled in the art. Moreover, said monitoring allows to use the most suitable biocide (or biocide mixture) according to the organisms detected thereby.

The method of the invention comprises (g.) optional steps of zooplankton control monitoring, subsequent to the (primary or secondary) treatments with biocides to test their effectiveness.

Such zooplankton monitoring step may be carried out monthly during the carrying out of the whole method, in order to provide a comprehensive knowledge of the species and of the number of individuals belonging thereto in the waters of interest, useful for ever more accurate forecasts of the zooplankton pattern at the site of interest.

The monitoring may be carried out according to:

-   a. Direct count of the larvae by sampling and filtering suitable     water volumes to search for larvae; -   b. Automatic count of the particles having size and morphology     referable to the larvae to be quantified.

A plant according to the present invention further comprises means for carrying out a qualitative and quantitative monitoring of the zooplankton near the surfaces to be controlled.

Such monitoring means comprises a filtering device comprising an on-deck suction pump and a cylinder containing one or more filters.

The cylinder is subdivided into plural segments, each having at one of its ends a filtration net of the same kind of those commonly used for plankton sampling.

These nets are available in various mesh port sizes and are made with an accuracy sufficing for the correct operation of the filtering device. According to an embodiment of the plant according to the present invention, the meshes are selected according to the size of the organisms that are to be netted.

In this case the filter is subdivided into three segments, their nets having 400 μm, 200 μm and 30 μm ports, respectively.

The segments are sequentially arranged to form a single cylinder through which the marine water sucked in by the pump is flown.

At the end of the filtering, the filters yield the samples that should be fixed to be preserved until laboratory microscope analysis.

Biocides, Treatments with Biocides and Operating Means of Said Treatments.

The biocides according to the present invention may be any one biocide (or biocide mixture) known to a person skilled in the art suitable for the eradication of the aquatic filtering organisms of the present invention. Said biocides may be used in the standard concentrations, them also known to a person skilled in the art (as indicated e.g. in U.S. Pat. No. 5,900,157 or in the manufacturer's specification). Said biocides or biocide mixtures comprise, yet are not limited to: peracetic acid, chlorine gas, sodium hypochlorite, calcium hypochlorite, chlorine dioxide, ozone, hydrogen peroxide, bromine, permanganates (e.g., potassium permanganate), persulfates (e.g., ammonium, alkali metals), perborates, quaternary ammonium salts, polyquaternary ammonium compounds, Endothal (Elf Atochem's EVAC®), metallic compounds (e.g. copper, zinc, potassium), Niclosamide (Bayer, salt of 2-amine-ethanol with 2,5-dichloro-4-nitro-salicyl-anilide), Bayluscide (Bayer, 2,5-dichloro-4-nitro-salicyl-anilide), bromine chloride (Stabrom 909, Albemarle), thiazoles and isothiazolines (Kathon WT, Rhom & Haas).

Any other biocide (o biocide mixture) apt to eradicate filtering organisms according to the present invention, used in suitable concentrations, may be useful for the purpouses of the present invention.

The treatments with said biocides, or biocide mixtures, of the waters near the submerged structures of interest, may be ‘primary’ or ‘secondary’. As primary treatments, in the meaning of the present invention, there are defined those treatments (one or more) that are carried out at the period of maximum density of the aquatic filtering organisms of the present invention, calculated on the basis of the data obtained at a. b. c. and d. of the above described method. As secondary treatments, in the meaning of the present invention, there are defined those treatments (one or more) that may be carried out at periods other than that of density growth of said filtering organisms, like e.g. at periods of minimum density of said organisms or intermediate between the minimum densities and the maximum density (see FIG. 1).

The primary treatments are carried out from 1 to 8 or from 1 to 4 times per maximum-density period, for a duration ranging from 12 to 240 hours or for a duration ranging from 48 to 160 hours or for a duration ranging from 48 to 100 hours. Said treatments are carried out with biocide (or biocide mixture) concentrations ranging from 300 to 0.5 ppm (μl/l). The biocide concentration used is higher for short-time treatments and lower for long-time treatments, e.g. it ranges from 0.5 to 10 ppm for treatments of about 240 hours and from 10 to 100 ppm for treatments of about 48 hours. Said treatments can be carried out from 1 to 4 times during the period of maximum density, for a duration ranging from 48 to 140 hours at a concentration ranging from 1.5 to 30 ppm.

In a further embodiment of the present invention, besides the primary treatments as described hereto, further secondary treatments are carried out at the periods of minimum density of said filtering organisms, from 1 to 8 times in each period of minimum density, for a duration ranging from 12 to 240 hours. The secondary treatments are carried out at biocide (or biocide mixture) concentrations lower with respect to the primary treatments, ranging from 80 to 0.5 ppm. Said treatments are carried out from 4 to 8 times per period of minimum density for a duration ranging from 48 to 240 hours at biocide (or biocide mixture) concentrations ranging from 0.5 to 20 ppm.

According to an embodiment, a plant according to the present invention further comprises means apt to allow the carrying out of the water treatments with biocides, primary as well as secondary ones.

Such operating means may comprise devices that, controlled by the plant managing unit, are apt to automatically carry out the required treatments, e.g. operating specific mechanisms like pumps, valves or anything else useful to this end.

A detailed specification of said devices and of their operation will be omitted, as the same lie within the capacity of a person skilled in the art.

Alternatively, such operating means may merely comprise acoustic and/or visual signalling devices informing an operator about the best time and modes for carrying out a certain treatment.

Salinity and/or Temperature Value Sampling and Sampling Means.

The method of the present invention optionally foresees the monitoring of the water temperature and/or salinity, concomitantly to the monitoring carried out at a., for integrating or correcting the values obtained at a. and their use. The temperature values may be integrated to the phytoplankton biomass values for the calculation of the time of maximum zooplankton density. In fact, the temperature directly influences phytoplankton and zooplankton development determining environmental conditions more or less suitable for said development. The temperature-related environmental conditions optimal for phytoplankton and zooplankton development are known in literature. Therefore, the integration of the sampled temperature values to identification parameters of phytoplankton biomass values, e.g. Chl-A, allows a more accurate forecasting of the period of maximum density of the filtering organisms according to the present invention.

Concerning the monitoring of the salinity values, when the waters of interest are salty or brackish said values may be used to correct the phytoplankton biomass values to derive their pattern, and, consequently, to calculate the period of maximum density of the filtering organisms. In fact, in the case of salty waters, salinity values allow to indirectly assess the entity of a possible exogenous phytoplankton biomass contribution due to freshwater currents of continental origin nearby the submerged surfaces of interest. Said assessment allows to analyse the variation in the salinity values of the waters of interest, relating them to possible rises in phytoplankton biomass values. Hence, said relationship allows to consider the phytoplankton biomass values actually involved in the time-pattern of said biomass related to the areas of interest, discriminating them from sporadic concentration rises due to freshwater inletting.

Moreover, advantageously the temperature, salinity and phytoplankton biomass values may be integrated for the calculation of the period of maximum density of said filtering organisms.

The sampling of salinity and/or temperature values according to the present invention may be ‘manually’ carried out weekly in the waters of interest, according to the phytoplankton sampling, and the related data obtained by simple laboratory procedures known to a person skilled in the art. According to the present invention, said values may be sampled also using electronic probes apt to meter one or both of said values.

Said samplings may be carried out by multiparameter probes, like, e.g., the CTD (Conductivity, Temperature, Depth) multiparameter probes. According to a further embodiment of the present invention, all data related to the pattern of the phytoplankton biomass and to the subsequent density values of filtering organisms detected over time, as well as all the meterings of fluorescence, temperature, salinity and of any other metered parameter, are stored in a historical archive.

Thus, such data may be used for statistical purpouses, during subsequent processing, intending to improve the accuracy of the estimates and of the forecasts carried out and with the final purpose of increasing the method effectiveness over time.

The plant on the invention also envisages the use of a computer program product, comprising a program code stored on a storage medium, the program being apt to cooperate with the plant previously described, such that the method according to one of the invention is carried out. Said computer program product, may comprise a program code for executing the method of the invention when the program is running on a computer.

The following methods and examples are meant to provide a more detailed description of the invention, yet without limiting what is being claimed thereto.

Experimental Procedures

Biomass Assessment by Average Biocellular Volume Calculation

Water sampling is carried out by automatic closure Niskin-type bottles of an approximate volume of 10 litres, equipped with a basal stopcock allowing subsample collecting. Water subsamples destined to phytoplankton analysis are stored in dark glass bottles (250 ml volume) to prevent chlorophyll decay and fixed with prefiltered 0.8% formaldehyde, neutralised and stabilised with hexamethylene tetramine at pH 7.0-7.2.

Then, the samples are laboratory analysed following the Utermöhl method (Utermöhl H., 1958. Zur Vervollkommnung der quantitativen Phytoplankton Methodik. Mitt. Int. Verein. Theor. Angew. Limnol., 9:1-38). The sample is manually agitated for a prolonged time to resuspend the phytoplankton cells settled on the bottle bottom, taking care to adopt slow and gentle motions in order not to injure the organisms.

The procedure foresees the collecting of a variable-volume water aliquot, depending on the phytoplankton density forecast from the features of the sampling site examined (e.g. distance from shore and depth). This aliquot is left to settle in suitable known-volume glass cylinders (10-100 ml), resting on known-radius circular-section settling chambers. The settling time varies according to the volume to be analysed and to the cylinder height: a waiting time of about 3-5 hours per cm of cylinder height is advisable. At the end of the settling, the cylinder is separated from the settling chamber by sliding on a suitable support, the sample is covered with a slide coverslip, avoiding bubble formation and settled material dispersion. Observation is carried out by phase-contrast inverted optical microscope with 200× and 400× magnification.

Phytoplankton cell count is conventionally carried out on transepts or on random visual areas, called fields. A transept corresponds to an area having a width equal to the diameter of the settling chamber and a height equal to the field of view of the lens used. The random visual fields are obtained by dividing the settling chamber bottom into a central circle and 9 circular concentric crowns, all having the same area. By intersecting these areas with 5 diameters, crossed at equal angles thereamong, 100 circular sectors (or segments) are obtained. Hence, for the count a certain number of fields (30-60) are observed, randomly drawn among the 100 segments obtained beforehand. The count obtained by the transept method tends to produce an overestimation in the central area of the chamber, therefore it is convenient to adopt the random field method.

Then the phytoplankton cell count values are reported to the number of individuals per ml (=cells/ml).

Instead, the estimate of the biomass of the entire phytoplankton community is obtained by calculating the average biocellular volumes. In fact, a metering of each single cell by a micrometric eyepiece is carried out in parallel with the cell count. Biovolume is estimated by geometric formulas, reporting each phytoplankton species to an analogous three-dimensional geometric shape. The cellular plasma volume thus estimated is used for calculating the phytoplankton biomass according to Strathmann formula (1967):

-   -   for the diatoms:     -   log₁₀ C=0.758 (log₁₀ V)−0.422 for the remaining phytoplankton         species:     -   log₁₀ C=0.866 (log₁₀ V)−0.460 where:     -   V=specific average cellular volume     -   C=μg organic carbon/cell

Therefore, the phytoplankton biomass values are expressed in μg C/1.

Spectrophotometric Method

In order to avail oneself of a pigment amount sufficing for the analysis, it is necessary to concentrate phytoplankton cells, present in the water sample (from 1 to 5 litres collected with Niskin-type bottles and prefiltered with a 200 μm net), by filtration on GF/F Whatman glass fibre filters using a <200 mm Hg suction pressure. The filters are stored at −20° C., in a sealed test pipe and in a known volume of neutralised acetone, for a ≦30-day period.

Chlorophyll A is cold-extracted from phytoplankton cells consequently to filter grinding and homogenising (by homogeniser or manual action with mortar and pestle) in a 90% acetone mixture. The test tubes, containing 10 ml of 90% acetone and the homogenate filter, are kept at 5° C. in the dark for about 24 hours, then a 3000-4000 rpm centrifugation is carried out for 10 min.

Then, the photosynthetic pigment concentration is estimated by spectrophotometric method.

The spectrophotometer should possess the following technical requirements: interference grid and 1-2 nm bandwidth, cells having a 50-100 mm optical path and a maximum volume of 7 ml. After centrifugation, the supernatant is collected and poured in the spectrophotometer cuvette. The readings are performed at 665 nm and at 750 nm wavelengths (the reading at 750 nm evaluates the sample turbidity; the absorbance value should be lower than 0.010, otherwise a further centrifugation or another filtration of the sample with a 0.2 μm filter will be required).

Subsequently, the analytic procedure foresees the acidification of the sample upon addition of 50 μl of HCl 1N per 5 ml of extract directly in the spectrophotometer cuvette. After having agitated the sample and waited about 1 min, the readings are repeated at the same wavelengths.

Then, the net absorbances of the extract, pre- and post-acidification ([A(665₀)] and [A(665_(α))] respectively), are determined according to the following formula: A(665_(α))=[A(s, 665_(α))−A(b, 665)]−[A(s, 750_(α))−A(b, 750)] where:

-   -   A(b, 665)=blank absorbance at 665 nm     -   A(b, 750)=blank absorbance at 750 nm     -   A(s, 665_(α))=sample absorbance at 665 nm, pre- (α=0) and         post-acidification (α=a)     -   A(s, 750_(α))=sample absorbance at 750 nm, pre (α=0) and         post-acidification (α=a)     -   Chl-A and pheopigment concentrations are calculated applying the         following formulas:         Chl-A(μg/l)=26.7* [A(665₀)−A(665_(a))]*v*10³/(CO*V)         Pheo (μg/l)=26.7*[1.7*A(665_(a))A(665₀)]*v*10³/(CO*V)         where:     -   CO=optical path of the cell (cm)     -   v=extract volume (ml)     -   V=filtered sample volume (ml)

Fluorimetric Method

For the estimation of the Chlorophyll-A, the present method is based on the metering of the fluorescence of the acetone extract pre- and post-HCl acidification, thereby metering the photosynthetically active (chlorophyll A) and inactive (pheopigments) fraction of the pigments in the sample.

This method is particularly suitable for concentration values of pigments in water column not higher than 1 μg/l, as above these values the fluorescence/concentration relationship is not held linear anymore.

The spectrofluorimeter is selected on the excitation and emission bands proper of chlorophyll-A: EXC=430 nm and EMISS=665 nm. After having reset the instrument with a blank (90% acetone), the extract fluorescence (FO) is metered. Subsequently, 2 drops of HCl 1N are added with a Pasteur pipette and, after about 1 min, the post-acidification fluorescence value (F_(a)) is metered.

To calculate the concentration of the chlorophyll-A and of the pheopigments, the following formulas are employed: mg Chl-A l⁻¹ =R/(R−1)*(F ₀ −F _(a))*C*(v/V) mg Pheo l⁻¹ =R/(R−1)*((R*F _(a))−F ₀)*C*(v/V) where:

-   -   F₀=pre-acidification fluorescence     -   F_(a)=post-acidification fluorescence     -   C=mean factor C_(a)/F_(0 st.) calculated for the standards     -   Ca=known concentration of each individual standard     -   F_(0 st.)=fluorescence of the standard relative to the         pre-acidification concentration C_(a)     -   R=mean factor F_(0 st.)/F_(a st.) calculated for the standards     -   F_(a st.)=fluorescence of the standard relative to the         post-acidification concentration C_(a)     -   V=acetone extract volume (ml)     -   V=filtered sample volume (1)

High-Resolution Chromatographic Method (HPLC)

The analytical procedure foresees the extraction of the pigments with acetone and methanol; the extracts are then concentrated and separated in a 30-cm column (Partisil-10 with 4600 theoretical plates) by means of a mobile phase of petrol ether, acetone, dimethyl sulfoxide and diethylamine, dosed with a 2 cm³/min flow at a 14-atm pressure.

Spectrophotometer at a 440 nm Wavelength.

The method, highly sensitive, allows the detection of 80 ng/l Chl-A concentrations.

Fluorimetric Method on in vivo Suspensions

This method is mainly based on fluorescence metering in the red band (emission spectral maximum: 680±2 nm) emitted by the Chlorophyll A and by the accessory pigments consequently to blue-light excitation (emission spectral maximum: 432±4 nm) . The water sample is pumped through a quartz pipe mounted inside the fluorimetric probe along the longitudinal axis. The pigment chlorophyll, present in the water-suspended phytoplankton cells, excited by an external light source (blue light) absorbs light in a certain region of the visible spectrum, remitting a light fraction as fluorescence at a higher wavelength (red light). To this end, LEDs emit light in the blue band (excitation), whereas a detector, equipped with a silicon photodiode, located at 900 with respect to the axis of the LEDs, meters the fluorescence emitted by the water volume under exam.

Then, the fluorescence values are converted in Chl-A concentration (in absolute value) directly from the instrument, gauged with pure chlorophyll standards directly by the manufacturer.

The high sensitivity of the commercially available fluorimetric probes allows to estimate Chl-A over a wide range of concentrations, of from 0.01*10⁻³ to 0.01 mg/l.

The use in situ of fluorimeters associated to CTD multiparameter probes allows a real-time and continuous detection of the chlorophyll-A concentration in water column, further allowing to highlight both its vertical and its horizontal distribution.

EXAMPLE I

Use of Peracetic Acid as Biocide for the Bivalve Mussel Mytilus galloprovincialis

Peracetic acid, also known as peroxyacetic acid (CH₃CO₃H), is a weak acid commonly commercialised as antibacterial agent for water and food industry disinfection. The commercial product is an equilibrium mixture in aqueous solution of peracetic acid, acetic acid, hydrogen peroxide and stabilisers (phosphates, phosphonic acids, dipicolinic acids).

The reaction at balance is the following: CH₃CO₃H+H₂O CH₃CO₂H+H₂O₂

The decomposition of the product generates methane, carbon dioxide, oxygen and water, all ecocompatible products entailing an extremely low environmental impact.

Commercial peracetic acid solution is available in concentrations ranging from about 1 to about 40 wt %, though diluted solutions of 1-15 wt % peracetic acid are commonly used, since at higher concentrations it exhibits a strong corrosive action.

In the case of higher Metazoans, the product was effectively tested on the bivalve mollusc Dreissena polymorpha (zebra mussel), a freshwater species infesting many lakes and rivers of the United States and of Canada. This organism is deemed to be the main culprit of the damaging to the conduction, filtering and cooling (CWS) systems in industrial plants having heat exchangers, equipped with water inlets directly from the water body.

Several other reagents may be used in combination with the peracetic acid in order to attain a synergic effect, with the entailed reduction of the treatment times and costs. These reagents comprise other above described biocides (molluscicides), oxidising and not oxidising, as well as mere additives capable of increasing the oxidising activity of the peracetic acid.

In some cases the use of surfactants (anionic, cationic or nonionic, among which anionic sulfates and sulfonates) in combination with the peracetic acid may prove convenient in order to facilitate biocide dispersion, above all in the presence of foam preventing contact between the biocide and the organism to be eradicated.

Tested Products

In the case at issue, different concentrations of peracetic acid (PAA) at a nominal concentration of 150 g/L (15 wt %) produced by Ausimont were tested.

The biocidal action of different concentrations of bromine chloride (trade name: Stabrom) at a nominal concentration of 105-115 g/L (10.5-11.5 wt %) produced by Albemarle and of mixtures of the two products (PAA+Stabrom) in different ratios, was also tested.

Laboratory Procedures

Mytilus galloprovincialis specimens were sampled in the sea area in front of the Conero and left to acclimate in an aquarium for a 48-hour time.

The specimens were subdivided into two main biometric classes: 5-10 mm and >10 mm. The subdivision into size classes helped to assess the treatment effectiveness on organisms at a different development stage, since the resistance to the biocide is directly related to size.

The mussels are accommodated inside of suitable plastics material tanks equipped with a continuous aeration system. Inside each tank there are 10 mussels for each of the two size classes, whose reactivity and vitality is assessed immediately prior to the experiment. The solutions having different biocide concentrations are prepared using the water in the aquarium in order to avoid stresses due to a sudden temperature change. The temperature of the water in the aquarium ranges from 19 to 20° C. The mortality tests for each biocide concentration are conducted in triplicate; a set of three tanks is not subjected to any treatment (control blank) in order to assess whether the observed mortality be due exclusively to the action of the biocide or to the experimental conditions.

The mussels subjected to the treatment are observed over time, at 2-3-hour intervals, to their decease. Their behaviour is noted down, with the aim of determining the time in which the toxic agent proves effective in causing sublethal reactions. In particular, three different ethological parameters were considered, employed as stress bioindicators:

-   1) Visible mantle edges: the organism is vital and effecting regular     breathing and feeding processes. Under unfavourable environmental     conditions, mussels tightly clamp valves and discontinue any     exchange with the external environment. In the case of a prolonged     exposure to the biocide, a physiological response occurs as well:     due to the impossibility of effecting gas exchanges with the     outside, mussels are forced to set up an anaerobic metabolism that     entails the decalcification of the freshly-laid shell portions with     the consequent slowing down of the growth rate. -   2) Foot motion: The organism is actively searching for an optimal     position, and it rapidly reacts by retracting to foot inside the     valves when mechanically stimulated from the outside. Under     prohibitive environmental conditions the valve closure prevents the     coming out of the foot. -   3) Verticalization (adhesion to the substrate): the byssus protein     threads, secreted at the foot zone level, allow an effective     anchoring to the substrate with no energy expenditure. Under     environmental stress conditions, these threads are not secreted, or     are secreted in a number so limited as to ensure only a weak     adhesion to the substrate.

The organisms are considered dead when the valves remain open also after repeated mechanic stimulations of the mantle.

The (biocide-marine water) solution was renewed every 24 hours, as required in the case of ecotoxicological experiments having a constant concentration of biocide.

The biocide was dosed using a micropipette and the solution prepared using a graduated glass cylinder.

EXAMPLE II

Definition of the Sizes of the Larvae Entering the Plant

In order to proceed with the manual count as well as with the automatic count (operationally easier and methodologically safer as it provides a greater number of information over time) accurate information on Mytilus larvae size were used. In early life-cycle stages, the Mytilus specimens undergo a series of larval stages of increasing size (see Table 1). TABLE 1 Mytilus larval stages and sizes Common definitions Stage Size Duration Behaviour Larva Trochophore 1 dd “D” larva or Veliger 300 μm End of planktonic life prodissoconch (max) Post-larva Pediveliger Up to 7 wks Metamorphically competent; however, metamorphosis may be delayed until optimal substrate is found 225-470 μm — Certainly attributable to the Mytilus genus 250 μm-2 mm — Under static conditions, crawling and forming aggregates; not selecting a specific substrate and remaining at the original colonization site. 225-470 μm — Under turbulent conditions, attaching to a variety of substrates - preferentially to byssus threads and to Ceramium red algae thalli. 250 μm-2 mm — Attaching to filamentous algae. Post-larva Early — 1 mth Detaching again, drifting in the current; plantigrades Late  11 mm — Final colonization in a mussel bed plantigrades

The meshes of the nets belonging to the filtering system were selected according to the sizes of the different larval types (Trochophora, Veliger, Pediveliger). In fact, the larval forms of Mytilus, when exposed to a light of suitable wavelength at the light microscope, are highlighted by the appearance of an X allowing their immediate identification. In order to carry out the automatic count, also a particle counter (Coulter Counter) may be applied to the CTD for counting the larvae in situ. The apparatus is capable of emitting the desired light, and hence, set on the recognition of this detail, it allows to continuously count the larvae directly (and in situ).

Example III

Monitoring of the Waters Surrounding an Industrial Plant

A yearly monitoring of the waters surrounding an industrial plant using seawater for the alimentation of a series of once-through exchangers has been performed. The parameters monitored where: water temperature, chlorophyll-A concentration, zooplankton larvae counting was performed with the method described in paragraph “Zooplankton monitoring and means for carrying out said monitoring.” i.e. direct count of larvae after samling and filtering.

The Chl-A and temperature measurements have been performed using the multiparameter probe as described below:

-   Multiparameter probe ver.0.73 (IP031A)-IDROMAR TIME DELAYED -   Storing Range=02:00:00 (hh:mm:ss) -   Sampling Period=00:30 (mm:ss) -   Sampling Range=00:01 (mm:ss) -   Pre-ignition Delay=02 (ss)

Sensors: P (dbar) Pressure T (° C.) Temperature C (mS/cm) Conductibility Cl (μg/L) Chlorophyll-A Tb (V) Battery tension S (ppt) Salinity St (Kg/m3) Relative Density

For the larval counting, sampling and filtering were performed on 1000 L of water with three filters; the filters diameter was 70 mm and their nets having 20 μm-200 ports.

The results are described in FIG. 5 panels a. and b. Panel 5 a. shows the number of veliger larvae/liter of sea water (left axis of ordinates, bar graph) and the values of Chl-A measured by the probe in micrograms/liter (right axis of ordinates, linear graph) against time measured in months (axis of abscissas). The Chl-A is monitored in a continuous mode (with intervals of two hours) by the probe. From the graph the delay of the larval peak with respect to the Chl-A peak is evident. Panel 5 b. shows the number of veliger larvae/liter of sea water (left axis of ordinates, bar graph) and the values of Temperature measured by the probe in micrograms/liter (right axis of ordinates, linear graph) against time measured in months (axis of abscissas). The temperature is monitored in a continuous mode (with intervals of two hours) by the probe. The data shown in FIG. 5 (panels a and b) are summarised in Table 2. TABLE 2 Cl (μg/L) T (° C.) Data Average Cl Average T n° veliger/L July 1.55 25.46 17.8 August 1.73 26.52 7.5 September 2.97 25.02 3.7 October 1.64 20.89 10.1 November 1.40 17.98 3.7 Dicember 15.37 January 13.48 February 2.20 11.58 March 6.46 13.02 3.9 April 7.04 14.53 4.1 May 4.57 17.29 13.3 June 3.10 21.60

From the two graphs it is evident that the Chl-A patterns are striclty related (co-linear) and anticipate the larval growth patterns whereas the temperature patterns do not anticipate the larval growth patterns and are sometimes not co-linear.

The above mentioned results prove that, due to the fact that the Chl-A peaks anticipate in a linear way the zooplankton peaks, Chl-A values are valid parameters identifying phytoplankton biomass values in waters and, hence, allow the anticipation and the forecast of the time periods of larval proliferation. Said possibility of a reliable anticipation and forecast of the zooplankton growth pattern and of the zooplankton growth peaks allows, hence, to perform the control and/or prevention methods of the present invention. Parameters identifying phytoplankton biomass values in waters such as Chl-a values, hence, permit the use of the above mentioned biocides (or mixture thereof) in specific critical moments of larval growth in order to avoid colonisation and subsequent fouling due to said organisms of submerged structures.

The temperature pattern and the temperature growth usually is concomitant to the growth peaks, in some cases, however, the temperature pattern (September November) appears to be not directly connected to the larval growth.

Temperature, hence, cannot be the only parameter used to control larval growth and filtering organisms' colonisation.

The present example, therefore, proves that the simple reference to seasons (temperature) for the control of larval growth and filtering organisms' colonisation is, as stated in the state of the art, insufficient to perform a control method restricted to critical growth peaks and cannot be used to plan a specific control (time and shock intensity) with biocides.

On the other hand, a parameter identifying biomass values, such as in the present example Chl-A, is demonstrated to be a very useful and reliable parameter for the control and/or prevention of the colonization of submerged structure surfaces by aquatic filtering organisms as described by the present invention. 

1-26. (canceled)
 27. A method for controlling or preventing the colonization of submerged structure surfaces by aquatic filtering organisms, comprising the following steps: a. monitoring parameters identifying phytoplankton biomass values in waters near said surfaces; b. determining from said values a time-pattern of the phytoplankton biomass and identifying a time instant at which said pattern exhibits a peak, and; c. monitoring zooplankton in said time instant and assessing a presence or an absence of zooplankton-stage larvae of said filtering organisms near said surfaces; d. if the presence of said larvae is assessed, calculating as a function of said time instant a period of maximum density of said filtering organisms and, e. after said calculating, subjecting said waters to at least one primary treatment with at least one biocide during said period.
 28. The method according to claim 27, wherein the monitoring at a. may be carried out by a manual sampling, by an automatic monitoring, or by both.
 29. The method according to claim 27, wherein the monitoring at a. comprises metering of Chlorophyll-A concentration values.
 30. The method according to claim 29, wherein said monitoring comprises a step of converting the Chlorophyll-A concentration values into phytoplankton biomass values.
 31. The method according to claim 29, wherein said metering of Chlorophyll-A concentration is carried out by a fluorescence metering.
 32. The method according to claim 27, wherein said waters are marine or brackish, further comprising monitoring the salinity values of said waters concomitantly to the monitoring at a. and correcting the values identified at a. on the basis of the monitored salinity values.
 33. The method according to claim 27, wherein said at least one treatment is carried out from 1 to 8 times, for a duration ranging from 12 to 240 hours, at a concentration of said at least one biocide ranging from 300 to 0.5 ppm.
 34. The method according to claim 27, additionally comprising further monitoring of zooplankton.
 35. The method according to claim 27, additionally comprising calculating periods of minimum density of said filtering organisms and subjecting said waters to at least one secondary treatment with at least one biocide during at least one of said periods of said minimum density.
 36. The method according to claim 35, wherein said at least one secondary treatment is carried out from 1 to 8 times, for a duration ranging from 12 to 240 hours at a concentration of said at least one biocide ranging from 80 to 0.5 ppm.
 37. The method according to claim 27, wherein said filtering organisms are at the benthos stage during the at least one treatment with the at least one biocide.
 38. A plant for controlling the colonization of submerged structure surfaces by aquatic filtering organisms, comprising: a. means for monitoring parameters identifying phytoplankton biomass values in the waters near said surfaces; b. means for processing said values; c. means for qualitatively and quantitatively monitoring the zooplankton near said surfaces; d. operating means apt to carry out treatments of said waters with biocides.
 39. The plant according to claim 38, further comprising units for collecting and managing the values detected by the means at a. and further processing means apt to integrate the values processed by the means at step a. to the data detected by the means at step c.
 40. The plant according to claim 38, wherein the monitoring means at a. comprises a probe apt to monitor chlorophyll-A concentration values.
 41. The plant according to claim 40, wherein said probe is a fluorescence probe.
 42. The plant according to claim 38, wherein the monitoring means at a. further comprises a probe apt to detect water salinity.
 43. The plant according to claim 38, wherein the means at c. comprises a water filtering device apt to monitor in real-time the presence, the type and the density of the zooplankton.
 44. The plant according to claim 43, wherein said filtering device comprises a suction pump.
 45. The plant according to claim 43, wherein said filtering device comprises a filtering cylinder subdivided into two or more sequentially arranged segments.
 46. The plant according to claim 45, wherein each of said segments contains at least one filtration net located at one of its ends.
 47. The plant according to claim 46, wherein each of those filtration nets has a respective port of sizes apt to let through organisms of predefined dimensions.
 48. The plant according to claim 39, wherein said units for collecting and managing are apt to automatically receive said sampled values.
 49. The plant according to claim 38, wherein the processing means at b. are apt to process the monitored values to obtain corresponding phytoplankton biomass values and to determine the time-pattern of said biomass.
 50. The plant according to claim 49, wherein said processing means are apt to identify a time instant at which said pattern has a maximum or a minimum peak.
 51. The plant according to claim 50, comprising further processing means apt to integrate the values processed by the means at step a. to the data detected by the means at step c., wherein said further processing means are apt to extrapolate, on the basis of the time instant in which said pattern has a maximum or a minimum peak and on the basis of the data obtained by the monitoring means at step c., periods of maximum or of minimum density of said filtering organisms.
 52. The processing means according to claim 38, comprising one or more elaborating computer programs.
 53. The further processing means according to claim 39, comprising one or more elaborating computer programs.
 54. A computer program product, comprising program code stored on a storage medium, the program being apt to cooperate with a plant according to claim 38, such that a method according to claim 27 is carried out.
 55. A computer program product, comprising program code stored on a storage medium, the program being apt to cooperate with a plant according to claim 39, such that a method according to claim 53 is carried out.
 56. A computer program product, comprising program code for executing the method according to claim 53 when the program is running on a computer.
 57. A method for controlling or preventing the colonization of submerged structure surfaces by aquatic filtering organisms, comprising the following steps: a. monitoring parameters identifying phytoplankton biomass values in waters near said surfaces; b. determining from said values a time-pattern of the phytoplankton biomass and identifying a time instant at which said pattern exhibits a peak, and; c. monitoring zooplankton in said time instant and assessing a presence or an absence of zooplankton-stage larvae of said filtering organisms near said surfaces; d. if the presence of said larvae is assessed, calculating as a function of said time instant a period of maximum density of said filtering organisms and, e. after said calculating, subjecting said waters to at least one primary treatment with at least one biocide during said period; f. calculating periods of minimum density of said filtering organisms and subjecting said waters to at least one secondary treatment with at least one biocide during at least one of said periods of said minimum density; and g. further monitoring of zooplankton. 